Academic journal article Genetics

A Screen for Genes Regulating the Wingless Gradient in Drosophila Embryos

Academic journal article Genetics

A Screen for Genes Regulating the Wingless Gradient in Drosophila Embryos

Article excerpt

ABSTRACT

During the development of the Drosophila embryonic epidermis, the secreted Wingless protein initially spreads symmetrically from its source. At later stages, Wingless becomes asymmetrically distributed in a Hedgehog-dependent manner, to control the patterning of the embryonic epidermis. When Wingless is misexpressed in engrailed cells in hedgehog heterozygous mutant embryos, larvae show a dominant phenotype consisting of patches of naked cuticle in denticle belts. This dose-sensitive phenotype is a direct consequence of a change in Wg protein distribution. We used this phenotype to carry out a screen for identifying genes regulating Wingless distribution or transport in the embryonic epidermis. Using a third chromosome deficiency collection, we found several genomic regions that showed a dominant interaction. After using a secondary screen to test for mutants and smaller deficiencies, we identified three interacting genes: dally, notum, and brahma. We confirmed that dally, as well as its homolog dally-like, and notum affect Wingless distribution in the embryonic epidermis, directly or indirectly. Thus, our assay can be used effectively to screen for genes regulating Wingless distribution or transport.

SECRETED signaling molecules play an essential role kj in patterning developing metazoans. Work from recent years shows that the signaling activity of these molecules is tightly regulated (FREEMAN 2000). Among various levels of regulation, controlling the distribution of ligands in a field of cells is necessary to establish stable gradients that robustly pattern developing tissues (VINCENT and DUBOIS 2002). The distribution of ligands could conceivably be influenced by many factors, including the concentration of receptors, the composition of the extracellular matrix, or the rate of recycling and degradation of the ligand following internalization. The best-documented case is the regulation of ligand distribution by receptors. For example, in Drosophila, Hedgehog (Hh) range of action is regulated by its receptor Patched: if the Hh-binding domain of Patched is mutated, the spread of Hh is extended in the wing imaginai disc (CHEN and STRUHL 1996). Another case is the morphogen Decapentaplegic, whose concentration gradient is regulated by the amount of its receptor Thickveins (LECUIT and COHEN 1998; TANIMOTO et al 2000). In addition to canonical receptors, low-affinity receptors such as heparan sulfate proteoglycans are likely to affect ligand distribution. For instance, in wing discs, mutants that impede the synthesis of heparan sulfate change Hh distribution (BELLAICHE et al 1998; THE et al 1999; TAKEI et al 2003; HAN et al 2004a,b).

Gradient formation also depends upon the mechanism by which signaling ligands move in the plane of epithelia. It is not yet clear what these mechanisms are, but two main modes of transport have been proposed (reviewed by VINCENT and DUBOIS 2002; GONZALEZGAITAN 2003): (1) facilitated diffusion, where ligands diffuse within the extracellular space, and this diffusion is modulated by cell-surface receptors, and (2) transcytosis, where ligands are transported along the plane of the epithelium by repeated cycles of endocytosis and recycling to the cell surface.

In this study, we searched for genes that influence the distribution or transport of the signaling ligand Wingless (Wg), the homolog of vertebrate Wnt-1 in Drosophila. In the embryonic epidermis, Wg acts at a short range to regulate the activity of target genes, whereas it behaves as a long-range morphogen in the wing imaginai disc (reviewed by SETO and BELLEN 2004). In the wing, Wg forms a stable gradient with a source localized at the boundary between ventral and dorsal compartments and triggers the expression of target genes at different distances from its source in a concentrationdependent manner (ZECCA et al. 1996; NEUMANN and COHEN 1997).

Both facilitated diffusion and transcytosis have been proposed to explain how Wg moves from cell to cell. …

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